Excitation module, laser oscillator, and laser amplifier

ABSTRACT

A pumping module is provided with a first square rod group including a first square rod ( 21 ) having an optical axis ( 26 ) and having a couple of heat sinking surfaces normal to a direction of y axis perpendicular to the optical axis ( 26 ) and a second square rod ( 22 ) having the optical axis ( 26 ) in common with the first square rod and having a couple of heat sinking surfaces normal to a direction of x axis perpendicular to the optical axis ( 26 ) and the direction of the y axis; a second square rod group including a third square rod ( 23 ) having the optical axis ( 26 ) in common with the first square rod and having a couple of heat sinking surfaces normal to the direction of the y axis and a fourth square rod ( 24 ) having the optical axis ( 26 ) in common with the first square rod and having a couple of heat sinking surfaces normal to the direction of the x axis; and a 90-degree polarization rotator ( 25 ) disposed between the first and second square rod groups and having the optical axis ( 26 ) in common with the first square rod, for rotating a polarization of laser light passing therethrough by 90 degrees.

FIELD OF THE INVENTION

[0001] The present invention relates to a pumping module that makeslaser light pass through a square rod that absorbs pump light incidentthereon so as to provide a gain for the laser light. It also relates toa laser oscillator that uses the pumping module and a laser amplifierthat uses the pumping module.

BACKGROUND OF THE INVENTION

[0002] In a square rod (rectangular rod or slab) formed in the form of asquare pillar, which is used as a laser medium for a solid state laserapparatus, it is easy to install a mechanism (metallic heat sink or thelike) for dissipating heat generated in the square rod from a couple oflateral surfaces facing each other which serve as heat sinking surfacesand it is therefore easy to dissipate the heat from the square rod,because the heat sinking surfaces are planar.

[0003] Furthermore, a square rod has a feature of easily providing laseroscillation of linear polarization by causing birefringence to occur inthe square rod in one direction because a temperature gradient isproduced only in a direction of heat sinking if ideal heat sinking iscarried out. Therefore, a pumping module that uses a square rod issuitable for spaceborne laser equipment which requires conductioncooling, laser equipment intended for laser machining which requireshigh average laser power, and so on.

[0004]FIG. 1 is a diagram showing the structure of a prior art pumpingmodule which is so constructed as to use a square rod. For example, thepumping module of FIG. 1 is disclosed in pp. 434 of the followingreference 1.

[0005] <Reference 1>

[0006] Springer Series in Optical Sciences Vol.1 “Solid-State LaserEngineering the Fourth Version”, written by Walter Koechner and printedby Germany Springer Co. in 1996

[0007] In FIG. 1, reference numeral 1 denotes a square rod, referencenumeral 1 a denotes a heat sinking surface of the square rod 1,reference numeral 1 b denotes another heat sinking surface which isperpendicular to the heat sinking surface 1 a of the square rod 1,reference numeral 2 denotes cooling water, reference numeral 3 denotes apump light source, and reference numeral 4 denotes an optical axis. InFIG. 1, y axis coincides with a direction of heat sinking, z axiscoincides with the optical axis 4, and x axis coincides with a directionperpendicular to the y axis and the z axis.

[0008] Next, a description will be made as to the operation of the priorart pumping module.

[0009] In the pumping module that uses the square rod 1 of FIG. 1, pumplight emitted out of the pump light source 3 is absorbed by the squarerod 1, and this results in generation of a gain. The pump light thusamplifies laser light that propagates in the direction of the opticalaxis. Heat generated in the square rod 1 is dissipated via the heatsinking surface 1 a in the direction of the y axis by the cooling water2.

[0010] In the prior art pumping module, because a temperature gradientis produced only in the direction of heat sinking when generated heat isideally dissipated from the heat sinking surface 1 a, the two axes (fastaxis and slow axis) of heat birefringence caused by the temperaturegradient appear in the direction of the y axis and in the direction ofthe x axis, respectively. Therefore, when laser light linearly polarizedin the direction of the y axis or x axis is incident upon the pumpingmodule of FIG. 1, the laser light can propagate within the square rod 1with the linear polarization being held, and therefore the loss in thecavity due to decrease in the extinction ratio can be reduced and thelaser oscillation of linear polarization can be facilitated.

[0011] However, the pumping module of FIG. 1 has the followingdrawbacks.

[0012] In other words, because a thermal lens effect according to thetemperature gradient is produced only in the direction of heat sinking(the direction of the y axis), and no thermal lens effect is produced inthe direction (the direction of the x axis) perpendicular to thedirection of heat sinking, the square rod 1 serves as a cylindrical lensthat provides a lens effect only in the direction of the y axis, andthat provides astigmatism for the laser light passing therethrough.Therefore, a problem encountered in the pumping module is that amechanism of compensating for astigmatism is needed when the pumpingmodule of FIG. 1 is used for such a laser apparatus as a laseroscillator or a laser amplifier, just as it is, and the optical systemstructure thus becomes complex.

[0013] The structure of a cavity that uses a pumping module having amodified square rod is disclosed, as a technique for solving thisproblem, in pp. 437 of the above-mentioned reference 1.

[0014]FIG. 2 is a diagram showing the structure of a laser oscillator towhich the prior art pumping module is applied. The same referencenumerals as shown in FIG. 1 denote the same components or likecomponents.

[0015] In FIG. 2, reference numeral 5 denotes a square rod whose bothends are ground so that they have a Brewster Angle, reference numeral 5a denotes a heat sinking surface of the square rod 5, reference numeral5 a denotes a lateral surface perpendicular to the heat sinking surface5 a of the square rod 5, reference numeral 6 denotes a total reflectionmirror, reference numeral 7 denotes a partial reflection mirror, andreference numeral 8 denotes an optical path along which laser lightpropagates within the laser oscillator.

[0016] Next, a description will be made as to the operation of the priorart laser oscillator.

[0017] In the laser oscillator of FIG. 2, because laser light propagatesalong a zig-zag optical path in the square rod 5, temperature gradientscaused in the direction of heat sinking are made uniform and thereforethe thermal lens effect can be compensated for. Furthermore, because nothermal lens effect is produced in the direction of the x axis when heatis ideally dissipated from the heat sinking surface 5 a, the astigmatismcan be compensated for.

[0018] However, because the square rod 5 actually used has a limitedsize in the case of using the pumping module shown in FIG. 2, heat isalso dissipated from the lateral surface 5 b through radiation andconduction even though heat is dissipated from the heat sinking surface5 a. Thus a problem encountered in the prior art laser oscillator isthat it is difficult for heat to be ideally dissipated only in thedirection of the y axis, and the direction of the temperature gradientsis not parallel to the direction of the y axis and therefore therecauses an inclination in the direction of the temperature gradients.

[0019] This problem will be explained a little more in detail.

[0020] FIGS. 3(a) and 3(b) are diagrams for explaining an inclination ofthe directions of the temperature gradients in the square rod 5 of thelaser oscillator of FIG. 2, and each of them shows a cross section ofthe square rod 5 taken along a plane perpendicular to the z axis. FIG.3(a) shows a case where heat is ideally dissipated from the heat sinkingsurface 5 a, and FIG. 3(b) shows a case where there causes variations inthe directions of the temperature gradients in the square rod 5 having alimited size. The same reference numerals as shown in FIG. 2 denote thesame components and like components.

[0021] In the case of FIG. 3(a), because heat is ideally dissipated onlyfrom the heat sinking surface 5 a, all the directions of the temperaturegradients become parallel to the y axis and the birefringence axesappear in the direction of y axis and in the direction of x axis,respectively, wherever in the cross section of the square rod 5.Therefore, when laser light linearly polarized in the direction of the yaxis or x axis is incident upon the square rod 5, the laser lightpropagates within the square rod 5 with the linear polarization thereofbeing held. Because the temperature gradients are produced only in thedirection of the y axis, a thermal lens effect is produced only in thedirection of the y axis and no thermal lens effect is produced in thedirection of the x axis.

[0022] On the other hand, even in the case of FIG. 3(b), when focused tothe center of the square rod 5 and line segments on A axis and B axis ofthe square rod 5, the directions of the temperature gradients areparallel to the y axis and the birefringence axes appear in thedirection of the y axis and in the direction of the x axis,respectively.

[0023] However, ununiformity is caused in the directions of thetemperature gradients produced in the cross section of the square rod 5because the square rod 5 has a limited size and therefore heat is alsodissipated from the lateral surface 5 b through radiation andconduction. In other words, when focused to other than the center of thesquare rod 5 and line segments on the A axis and B axis of the squarerod 5, the temperature gradients are inclined against the direction ofthe y axis.

[0024] Therefore, there cause variations in the orientation of the twobirefringence axes in cross section of the square rod 5. When laserlight linearly polarized in the direction of the y axis or the x axispasses through the square rod 5 of FIG. 3(b), a decrease occurs in thedegree of linear polarization and hence a decrease occurs in theextinction ratio due to the ununiformity of the birefringence.Furthermore, when the pumping module of FIG. 2 is applied to such alaser apparatus as a laser oscillator or a laser amplifier, a decreasein the efficiency of energy and a decrease in the beam quality canoccur.

[0025] Moreover, temperature gradients are produced in the direction ofthe x axis and a weak thermal lens effect is produced because thedirections of the temperature gradients have an inclination with respectto the y axis. This means that astigmatism occurs due to the thermallens effect produced in the direction of the x axis while the thermallens effect in the direction of the y axis can be canceled in the priorart pumping module as shown in FIG. 2. Therefore, a problem with theprior art pumping module is that a mechanism of compensating forastigmatism is needed for such a laser apparatus to which the pumpingmodule of FIG. 2 is applied as a laser oscillator or a laser amplifier,and the optical system structure thus becomes complex.

[0026] A problem with prior art pumping modules constructed as mentionedabove is that the extinction ratio of laser light passing through thesquare rod is decreased because it is difficult to provide ideal heatsinking such that temperature gradients are produced only in thedirection of the y axis and this results in occurrence of a variation inthe orientation of the birefringence axes.

[0027] Another problem with prior art pumping modules is thatastigmatism occurs because a thermal lens effect is also produced in thedirection of the x axis.

[0028] Another problem encountered in such a laser apparatus to which aprior art pumping module is applied as a laser oscillator or a laseramplifier is that a decrease in the efficiency of energy and a decreasein the beam quality can occur, and a mechanism of compensating forastigmatism is needed and the optical system structure thus becomescomplex.

[0029] The present invention is proposed to solve the above-mentionedproblems, and it is therefore an object of the present invention toprovide a pumping module capable of preventing any decrease in theextinction ratio which is caused by a variation in the orientation ofthe birefringence axes that occurs in a square rod, and reducingastigmatism.

[0030] It is another object of the present invention to provide a laseroscillator and a laser amplifier capable of preventing any decrease inthe efficiency of energy and any decrease in the beam quality withouthaving to use a mechanism for compensating for astigmatism.

DISCLOSURE OF THE INVENTION

[0031] A pumping module in accordance with an aspect of the presentinvention is provided with a first square rod group including a firstsquare rod having an optical axis and having a couple of heat sinkingsurfaces normal to a first axis perpendicular to the optical axis and asecond square rod having the optical axis in common with the firstsquare rod and having a couple of heat sinking surfaces normal to asecond axis perpendicular to the optical axis and the first axis; asecond square rod group including a third square rod having the opticalaxis in common with the first square rod and having a couple of heatsinking surfaces normal to the first axis and a fourth square rod havingthe optical axis in common with the first square rod and having a coupleof heat sinking surfaces normal to the second axis; and a 90-degreepolarization rotator disposed between the first and second square rodgroups and having the optical axis in common with the first throughfourth square rods, for rotating a polarization of the laser lightpassing therethrough by 90 degrees.

[0032] As a result, the present invention offers an advantage of beingable to prevent any decrease in the extinction ratio regardless of avariation in the orientation of the birefringence axes of each of theplurality of square rods, and to prevent the occurrence of a differencebetween a thermal lens with respect to the direction of the first axisand another thermal lens with respect to the direction of the secondaxis, thereby preventing the occurrence of astigmatism.

[0033] In the pumping module in accordance with another aspect of thepresent invention, the first square rod group is provided with the firstsquare rod and the second square rod in each of which the laser light isallowed to propagate along a zig-zag optical path between the couple ofheat sinking surfaces thereof, and the second square rod group isprovided with the third square rod and the fourth square rod in each ofwhich the laser light is allowed to propagate along a zig-zag opticalpath between the couple of heat sinking surfaces thereof.

[0034] As a result, the present invention offers another advantage ofbeing able to make temperature gradients caused in a direction of heatsinking uniform in each of the plurality of square rods, therebypreventing thermal lens effects from being produced.

[0035] In accordance with another aspect of the present invention, apumping module is provided with a plurality of pumping modules accordingto claim 1 and the plurality of pumping modules are arranged so thattheir optical axes coincide with one another and they are cascaded.

[0036] As a result, the present invention provides another advantage ofbeing able to increase the number of square rods, thereby increasing thegain to be given to the laser light.

[0037] In the pumping module in accordance with a further aspect of thepresent invention, the first square rod group is provided with the firstsquare rod and the second square rod which are integrally formed, andthe second square rod group is provided with the third square rod andthe fourth square rod which are integrally formed.

[0038] As a result, the present embodiment provides another advantage ofbeing able to facilitate the alignment of each of the plurality ofsquare rods and to omit processes such as grinding and coating of eachof the plurality of square rods, thereby reducing the cost of thepumping module.

[0039] In accordance with another aspect of the present invention, apumping module is provided with: a first polarization rotating opticalsystem including seventh and eighth square rods having an optical axisand each having a couple of heat sinking surfaces normal to a first axisperpendicular to the optical axis, and a first 90-degree polarizationrotator having the optical axis in common with the seventh and eighthsquare rods and disposed between the seventh and eighth square rods, forrotating a polarization of the laser light passing therethrough by 90degrees; and a second polarization rotating optical system includingninth and tenth square rods having the optical axis in common with theseventh and eighth square rods and each having a couple of heat sinkingsurfaces normal to a second axis perpendicular to the optical axis andthe first axis, and a second 90-degree polarization rotator having theoptical axis in common with the seventh and eighth square rods anddisposed between the ninth and tenth square rods, for rotating apolarization of the laser light passing therethrough by 90 degrees.

[0040] As a result, the present invention offers an advantage of beingable to prevent any decrease in the extinction ratio regardless of avariation in the orientation of the birefringence axes of each of theplurality of square rods, and to prevent the occurrence of a differencebetween a thermal lens with respect to the direction of the first axisand another thermal lens with respect to the direction of the secondaxis, thereby preventing the occurrence of astigmatism.

[0041] In the pumping module in accordance with a further aspect of thepresent invention, the first polarization rotating optical system isprovided with the seventh square rod and the eighth square rod in eachof which the laser light is allowed to propagate along a zig-zag opticalpath between the couple of heat sinking surfaces thereof, and the secondpolarization rotating optical system is provided with the ninth squarerod and the tenth square rod in each of which the laser light is allowedto propagate along a zig-zag optical path between the couple of heatsinking surfaces thereof.

[0042] As a result, the present invention offers another advantage ofbeing able to make temperature gradients caused in a direction of heatsinking uniform in each of the plurality of square rods, therebypreventing thermal lens effects from being produced.

[0043] In accordance with another aspect of the present invention, apumping module includes a plurality of pumping modules according toclaim 5 and the plurality of pumping modules are arranged so that theiroptical axes coincide with one another and they are cascaded.

[0044] As a result, the present invention provides another advantage ofbeing able to increase the number of square rods, thereby increasing thegain to be given to the laser light.

[0045] In accordance with a further aspect of the present invention, apumping module is provided with: a reflection square rod group includingone or more eleventh square rods having an optical axis and each havinga couple of heat sinking surfaces normal to a first axis perpendicularto the optical axis and a same number of twelfth square rods as that ofeleventh square rods, having the optical axis in common with the one ormore eleventh square rods and each having a couple of heat sinkingsurfaces normal to a second axis perpendicular to the optical axis andthe first axis; a first total reflection mirror for reflecting the laserlight emitted out of the reflection square rod group towards thereflection square rod group; and a first 45-degree polarization rotatorhaving the optical axis in common with the one or more eleventh squarerods and disposed between the reflection square rod group and the firsttotal reflection mirror, for rotating a polarization of the laser lightpassing therethrough by 45 degrees.

[0046] As a result, the present invention offers an advantage of beingable to prevent any decrease in the extinction ratio regardless of avariation in the orientation of the birefringence axes of each of theplurality of square rods, and to prevent the occurrence of a differencebetween a thermal lens with respect to the direction of the first axisand another thermal lens with respect to the direction of the secondaxis, thereby preventing the occurrence of astigmatism.

[0047] In the pumping module in accordance with another aspect of thepresent invention, the reflection square rod group is provided with theone or more eleventh square rods and the one or more twelfth square rodsin each of which the laser light is allowed to propagate along a zig-zagoptical path between the couple of heat sinking surfaces thereof.

[0048] As a result, the present invention offers another advantage ofbeing able to make temperature gradients caused in a direction of heatsinking uniform in each of the plurality of square rods, therebypreventing thermal lens effects from being produced.

[0049] In the pumping module in accordance with a further aspect of thepresent invention, equal numbers of the one or more eleventh square rodsand the one or more twelfth square rods are integrally formed in thereflection square rod group.

[0050] As a result, the present embodiment offers an advantage of beingable to facilitate the alignment of each of the plurality of square rodsand to omit processes such as grinding and coating of each of theplurality of square rods, thereby reducing the cost of the pumpingmodule.

[0051] In accordance with another aspect of the present invention, alaser oscillator is provided with a pumping module according to claim 1;a total reflection mirror that is perpendicular to an optical axis ofthe pumping module; and a partial reflection mirror that is disposed sothat the pumping module is sandwiched between the partial reflectionmirror and the total reflection mirror and that is perpendicular to theoptical axis.

[0052] As a result, the present invention provides another advantage ofbeing able to prevent any decrease in the efficiency of energy and anydecrease in the beam quality without having to use a mechanism ofcompensating for astigmatism.

[0053] In accordance with a further aspect of the present invention, alaser oscillator is provided with: a pumping module according to claim5; a total reflection mirror that is perpendicular to an optical axis ofthe pumping module; and a partial reflection mirror that is disposed sothat the pumping module is sandwiched between the partial reflectionmirror and the total reflection mirror and that is perpendicular to theoptical axis.

[0054] As a result, the present invention provides another advantage ofbeing able to prevent any decrease in the efficiency of energy and anydecrease in the beam quality without having to use a mechanism ofcompensating for astigmatism.

[0055] In accordance with another aspect of the present invention, alaser oscillator is provided with: a pumping module according to claim8; and a partial reflection mirror that pairs up with a first totalreflection mirror of the pumping module and is disposed so that thereflection square rod group and the first polarization rotator aresandwiched between the first partial reflection mirror and the partialreflection mirror and that is perpendicular to the optical axis.

[0056] As a result, the present invention provides another advantage ofbeing able to prevent any decrease in the efficiency of energy and anydecrease in the beam quality without having to use a mechanism ofcompensating for astigmatism.

[0057] In accordance with a further aspect of the present invention, thelaser oscillator includes a second 45-degree polarization rotator havingthe optical axis in common with the pumping module and disposed betweenthe partial reflection mirror and the pumping module, for rotating apolarization of laser light passing therethrough by 45 degrees.

[0058] As a result, the present invention provides another advantage ofbeing able to prevent any decrease in the efficiency of energy and anydecrease in the beam quality without having to use a mechanism ofcompensating for astigmatism.

[0059] In accordance with another aspect of the present invention, thelaser oscillator includes a polarizer disposed on the optical axisbetween the partial reflection mirror and the pumping module, forallowing laser light of a predetermined polarization to passtherethrough, and for reflecting laser light of a polarizationperpendicular to the former laser light of predetermined polarization ina direction perpendicular to the optical axis, and a second totalreflection mirror for reflecting the laser light reflected by thepolarizer towards the polarizer.

[0060] As a result, the present invention provides another advantage ofbeing able to prevent any decrease in the efficiency of energy and anydecrease in the beam quality without having to use a mechanism ofcompensating for astigmatism.

[0061] In accordance with a further aspect of the present invention, alaser amplifier amplifies and outputs an input laser light by using apumping module according to claim 1.

[0062] As a result, the present invention provides another advantage ofbeing able to prevent any decrease in the efficiency of energy and anydecrease in the beam quality without having to use a mechanism ofcompensating for astigmatism.

[0063] In accordance with another aspect of the present invention, alaser amplifier amplifies and outputs an input laser light by using apumping module according to claim 5.

[0064] As a result, the present invention provides another advantage ofbeing able to prevent any decrease in the efficiency of energy and anydecrease in the beam quality without having to use a mechanism ofcompensating for astigmatism.

[0065] In accordance with a further aspect of the present invention, alaser amplifier includes: a pumping module according to claim 8; and apolarizer disposed on an optical axis of the pumping module, forallowing laser light of a predetermined polarization to passtherethrough, and for reflecting laser light of a polarizationperpendicular to the former laser light of the predeterminedpolarization in a direction perpendicular to the optical axis, the laserlight of the predetermined polarization being input to the pumpingmodule by way of the polarizer.

[0066] As a result, the present invention provides another advantage ofbeing able to prevent any decrease in the efficiency of energy and anydecrease in the beam quality without having to use a mechanism ofcompensating for astigmatism.

BRIEF DESCRIPTION OF THE FIGURES

[0067]FIG. 1 is a diagram showing the structure of a prior art pumpingmodule;

[0068]FIG. 2 is a diagram showing the structure of a laser oscillator towhich the prior art pumping module is applied;

[0069]FIG. 3 is figure for explaining an inclination in the directionsof temperature gradients in a square rod of the laser oscillator of FIG.2;

[0070]FIG. 4 is a diagram showing the structure of a pumping moduleaccording to embodiment 1 of the present invention;

[0071]FIG. 5 is a diagram for explaining a Jones matrix of a firstsquare rod;

[0072]FIG. 6 is a diagram for explaining a Jones matrix of a secondsquare rod;

[0073]FIG. 7 is a diagram showing the structure of a pumping moduleaccording to embodiment 2 of the present invention;

[0074]FIG. 8 is a diagram showing the structure of a pumping moduleaccording to embodiment 3 of the present invention;

[0075]FIG. 9 is a diagram showing the structure of a pumping moduleaccording to embodiment 4 of the present invention;

[0076]FIG. 10 is a diagram showing the structure of a laser oscillatorto which the pumping module according to embodiment 4 of the presentinvention is applied;

[0077]FIG. 11 is a diagram showing the structure of a laser oscillatorto which the pumping module according to embodiment 4 of the presentinvention is applied;

[0078]FIG. 12 is a diagram showing the structure of a laser oscillatorto which the pumping module according to embodiment 4 of the presentinvention is applied; and

[0079]FIG. 13 is a diagram showing the structure of a laser amplifier towhich the pumping module according to embodiment 4 of the presentinvention is applied.

PREFERRED EMBODIMENTS OF THE INVENTION

[0080] In order to explain the present invention in greater detail, thepreferred embodiments will be described below with reference to theaccompanying figures.

[0081] Embodiment 1.

[0082]FIG. 4 is a diagram showing the structure of a pumping moduleaccording to embodiment 1 of the present invention. The pumping moduleis constructed of a plurality of square rods.

[0083] In FIG. 4, reference numeral 21 denotes a first square rod (firstsquare rod group), reference numeral 22 denotes a second square rod(first square rod group), reference numeral 23 denotes a third squarerod (second square rod group), reference numeral 24 denotes a fourthsquare rod (second square rod group), reference numeral 25 denotes a90-degree polarization rotator, and reference numeral 26 denotes anoptical axis which the first through fourth square rods 21 to 24 and the90-degree polarization rotator 25 have in common.

[0084] As shown in FIG. 4, the first square rod 21 has a couple of heatsinking surfaces which are normal to a direction of y axis (a directionof a first axis), and the fourth square rod 24 has a couple of heatsinking surfaces which are normal to the direction of the y axis, too.The second square rod 22 has a couple of heat sinking surfaces which arenormal to a direction of x axis (a direction of a second axis), and thethird square rod 23 has a couple of heat sinking surfaces which arenormal to the direction of the x axis, too. The 90-degree polarizationrotator 25 is placed between the second square rod 22 and the thirdsquare rod 23.

[0085] The first square rod 21 and the fourth square rod 24 are pumpedin much the same way, and temperature gradients and birefringence areproduced in much the same way in the first and fourth square rods 21 and24. Similarly, the second square rod 22 and the third square rod 23 arepumped in much the same way, and temperature gradients and birefringenceare produced in much the same way in the second and third square rods 22and 23.

[0086] Next, a description will be made as to the operation of thepumping module of embodiment 1 of the present invention.

[0087] Laser light travels along the optical axis 26 and passes throughthe first and second square rods 21 and 22 in succession, and is thenincident upon the 90-degree polarization rotator 25. The 90-degreepolarization rotator 25 emits the incident laser light to the thirdsquare rod 23 after rotating the polarization of the incident laserlight by 90 degrees. The laser light emitted out of the 90-degreepolarization rotator 25 passes through the third and fourth square rods23 and 24 in succession. Pump light emitted out of a pump light sourcenot shown in the figure is absorbed by the first through fourth squarerods 21 to 24 and a gain is produced in each of them, and the laserlight traveling in the direction of the optical axis 26 is amplified bythe first through fourth square rods 21 to 24.

[0088] Changes in the polarization of the laser light propagatingthrough the pumping module will be explained by using a Jones matrix.

[0089] In general, assuming that laser light incident upon an arbitraryoptical element has a polarized component E_(x) in the direction of thex axis and a polarized component E_(y) in the direction of the y axisand the optical element has a Jones matrix J, the polarized componentsE_(xout) and E_(yout) of the laser light emitted out of the opticalelement are given by the following equation (1). $\begin{matrix}{\begin{pmatrix}E_{xout} \\E_{y\quad {out}}\end{pmatrix} = {J \cdot \begin{pmatrix}E_{x} \\E_{y}\end{pmatrix}}} & {{Equation}\quad (1)}\end{matrix}$

[0090] The Jones matrix J of the optical element is a matrix having 2rows and 2 columns. The equation (1) shows that the incident polarizedlight vector (E_(x), E_(y))^(T) of the laser light (the superscript T isan operator indicating a transposition) is converted into the emergingpolarized light vector (E_(xout), E_(yout))^(T) by the action J of theoptical element.

[0091] The Jones matrix J_(B)(α, δ) of a birefringence optical elementis given by the following equation (2). $\begin{matrix}{{J_{B}( {\alpha,\delta} )} = {\begin{pmatrix}{\cos \quad \alpha} & {{- \sin}\quad \alpha} \\{\sin \quad \alpha} & {\cos \quad \alpha}\end{pmatrix}\begin{pmatrix}{\exp ( {\frac{\delta}{2}} )} & 0 \\0 & {\exp ( {{- }\frac{\delta}{2}} )}\end{pmatrix}\begin{pmatrix}{\cos \quad \alpha} & {\sin \quad \alpha} \\{{- \sin}\quad \alpha} & {\cos \quad \alpha}\end{pmatrix}}} & {{Equation}\quad (2)}\end{matrix}$

[0092] α is an angle between the fast axis of the birefringence opticalelement and the x axis and δ is a phase difference between the fast axisand the slow axis of the birefringence optical element, where in thebirefringence optical element, one of the two birefringence axes alongwhich the phase of light is advanced is referred to as the fast axis(phase advance axis) and the other one of them along which the phase oflight is delayed is referred to as the slow axis (phase delay axis).

[0093] In addition, because the Jones matrix of a polarization rotatoris a rotation matrix having 2 rows and 2 columns, the Jones matrixJ_(Rot) of the 90-degree polarization rotator 25 is a 90-degree rotationmatrix given by the following equation (3). $\begin{matrix}{J_{Rot} = {\begin{pmatrix}{\cos \frac{\pi}{2}} & {{- \sin}\frac{\pi}{2}} \\{\sin \frac{\pi}{2}} & {\cos \frac{\pi}{2}}\end{pmatrix} = \begin{pmatrix}0 & {- 1} \\1 & 0\end{pmatrix}}} & {{Equation}\quad (3)}\end{matrix}$

[0094] The Jones matrices J_(R21) to J_(R24) of the first through fourthsquare rods 21 to 24 will be extracted below based on the equation (2)hereafter.

[0095]FIGS. 5 and 6 are diagrams for explaining the Jones matricesJ_(R21) and J_(R22) of the first and second square rods 21 and 22,respectively, and show cross-sectional views of the first and secondsquare rods 21 and 22, which are taken along a plane normal to theoptical axis 26. In FIGS. 5 and 6, the fast axis F of the birefringenceproduced in each of the first and second square rods 21 and 22 is shownby an arrow.

[0096] As explained in “Background of the Invention”, because ideal heatsinking cannot be carried out in cross section of the first square rod21 of FIG. 5(a) and therefore an inclination occurs in the directions ofthe temperature gradients, there causes a variation in the orientationof the fast axis F of the birefringence so that the fast axis F is notnecessarily parallel to the y axis.

[0097] In other words, as shown in FIG. 5(b) that is an enlarged view ofthe central part of the first square rod 21, the fast axis F on thecenter of the first square rod 21, and the fast axes on A and B axes ofthe rod are parallel to the y axis. In contrast, as shown in FIG. 5(c),the fast axis F has an arbitrary angle θ₁ (i.e., π/2+θ₁ with respect tothe direction of the x axis) with respect to the direction of the y axisat any location apart from the center of the first square rod 21, andthe A and B axes of the rod.

[0098] Therefore, based on the equation (2) the Jones matrix J_(R21) ofthe first square rod 21 is given by the following equation (4).$\begin{matrix}{J_{R\quad 21} = {J_{B}( {{\frac{\pi}{2} + \theta_{1}},\delta_{1}} )}} & {{Equation}\quad (4)}\end{matrix}$

[0099] The Jones matrix J_(R22) of the second square rod 22 can besimilarly considered.

[0100] All fast axes F are not parallel to the x axis in the secondsquare rod 22, as shown in FIG. 6(a). In other words, while, as shown inFIG. 6(b), the fast axis F on the center of the second square rod 22,and the fast axes on the A and B axes of the rod are parallel to the xaxis, as shown in FIG. 6(c), the fast axis F has an arbitrary angle θ₂(i.e., π/2−θ₂ with respect to the direction of the y axis) with respectto the direction of the x axis at any location apart from the center ofthe second square rod 22, and the A and the B axes of the rod.Therefore, the Jones matrix J_(R22) of the second square rod 22 can begiven by the following equation (5).

J _(R22) =J _(B)(π−θ₂,δ₂)  Equation (5)

[0101] The Jones matrix J_(R24) of the fourth square rod 24 and theJones matrix J_(R23) of the third square rod 23 can be considered asfollows.

[0102] In other words, in accordance with this embodiment 1, because thefirst square rod 21 and the fourth square rod 24 are pumped in much thesame way, and the second square rod 22 and the third square rod 23 arepumped in much the same way, the temperature gradients and thebirefringence axes similarly appear in each of the first and fourthsquare rods and the temperature gradients and the birefringence axessimilarly appear in each of the second and third square rods. Therefore,the Jones matrix J_(R24) of the fourth square rod 24 and the Jonesmatrix J_(R23) of the third square rod 23 can be given by the equations(4) and (5), respectively (J_(R24)=J_(R21), J_(R23)=J_(R22)).

[0103] The Jones matrix J_(All) of the entire pumping module of FIG. 4is calculated according to the following equation (6) based on the Jonesmatrices shown in the above-mentioned equations (2) to (5).$\begin{matrix}\begin{matrix}{J_{A\quad l\quad l} = {J_{R\quad 24} \cdot J_{R\quad 23} \cdot J_{R\quad o\quad t} \cdot J_{R\quad 22} \cdot J_{R\quad 21}}} \\{= {{J_{B}( {{\frac{\pi}{2} + \theta_{1}},\delta_{1}} )} \cdot {J_{B}( {{\pi - \theta_{2}},\delta_{2}} )} \cdot \begin{pmatrix}0 & {- 1} \\1 & 0\end{pmatrix} \cdot {J_{B}( {{\pi - \theta_{2}},\delta_{2}} )} \cdot {J_{B}( {{\frac{\pi}{2} + \theta_{1}},\delta_{1}} )}}} \\{= \begin{pmatrix}0 & {- 1} \\1 & 0\end{pmatrix}}\end{matrix} & {{Equation}\quad (6)}\end{matrix}$

[0104] The equation (6) shows that the pumping module of this embodiment1 operates in such a manner that despite the bearing angle θ₁ of thefirst and fourth square rods 21 and 24, the bearing angle θ₂ of thesecond and third square rods 22 and 23, the phase differences δ₁ and δ₂,the emerging polarized light vector (E_(xout), E_(yout))^(T) must beperpendicular to the incident polarized light vector (E_(x), E_(y))^(T).

[0105] In other words, when the Jones matrix J_(All) of the equation (6)is made to act on the incident polarized light vector (E_(x), E_(y))^(T)of the equation (1), the emerging polarized light vector is (E_(xout),E_(yout))^(T)=(−E_(y), E_(x))^(T). The calculation of the inner product(dot product) of the incident polarized light vector and the emergingpolarized light vector yields (E_(x), E_(y))^(T)·(−E_(y),E_(x))^(T)=E_(x)(−E_(y))+E_(y)E_(x)=0. It is therefore understood thatthe incident polarized light vector (E_(x), E_(y))^(T) is alwaysperpendicular to the emerging polarized light vector (E_(xout),E_(yout)) despite the birefringence of each of the first through fourthsquare rods 22 to 24.

[0106] Thus, the pumping module as shown in FIG. 4 functions as a90-degree polarization rotator so as to rotate an arbitrary polarizationof incident laser light by 90 degrees and emit the laser light.Therefore, the pumping module can prevent any decrease in the extinctionratio regardless of a variation in the orientation of the birefringenceaxes of each of the plurality of square rods. Application of the pumpingmodule of FIG. 4 to a laser apparatus, such as a laser oscillator or alaser amplifier, can prevent any decrease in the energy efficiency ofthe laser apparatus and any decrease in the beam quality.

[0107] Next, compensation of thermal lens effects will be explained.

[0108] Laser light incident upon the pumping module of FIG. 4 passesthrough two thermal lenses (the second and third square rods 22 and 23)in which heat sinking is carried out in the direction of the x axis andtwo other thermal lenses (the first and fourth square rods 21 and 24) inwhich heat sinking is carried out in the direction perpendicular to thedirection of the x axis. In other words, the laser light passes throughthe two other thermal lenses (the first and fourth square rods 21 and24) in which heat sinking is carried out in the direction of the y axisand the two thermal lenses (the second and third square rods 22 and 23)in which heat sinking is carried out in the direction perpendicular tothe direction of the y axis.

[0109] Therefore, in the pumping module of FIG. 4 there is no differencebetween the thermal lenses in the direction of the x axis and thethermal lenses in the direction of the y axis, and astigmatism can beprevented from occurring. Application of the pumping module of FIG. 4 tosuch a laser apparatus as a laser oscillator or a laser amplifier, anymechanism for compensating for astigmatism becomes unnecessary andtherefore the optical system can be simplified.

[0110] In the above explanation, it is assumed that the laser lightpassing through the first through fourth square rods 21 to 24 propagatesin parallel with the optical axis 26. This embodiment 1 is not limitedto this configuration and each of the first through fourth square rods21 to 24 can have both ends each of which is so formed and ground as tohave a Brewster angle, like the square rod 5 of FIG. 2, so that thelaser light propagates along a zig-zag optical path which is bentseveral times in the direction of the y axis in the first square rod 21,along a zig-zag optical path which is bent several times in thedirection of the x axis in the second square rod 22, along a zig-zagoptical path which is bent several times in the direction of the x axisin the third square rod 23, and along a zig-zag optical path which isbent several times in the direction of the y axis in the fourth squarerod 24.

[0111] This configuration does not cause any thermal lens effect withrespect to the direction of heat sinking in each of the plurality ofsquare rods in each of which the incident laser light propagates along azig-zag optical path because the temperature gradients caused in thedirection of heat sinking are made uniform. Therefore, because as forthe direction of the x axis the incident laser light passes through athermal lens having a thermal lens effect with respect to a directionperpendicular to the direction of heat sinking twice, and, as for thedirection of the y axis, also passes through a thermal lens having athermal lens effect with respect to a direction perpendicular to thedirection of heat sinking twice, there causes no difference between thethermal lens with respect to the direction of the x axis and the otherthermal lens with respect to the direction of the y axis and theastigmatism can be compensated for.

[0112] In addition, the thermal lens of the entire pumping modulebecomes small because it doesn't receive the influence of the thermallens effect with respect to the direction of heat sinking, and thedesign and structure of such a laser apparatus to which the pumpingmodule is applied to, as a laser oscillator or a laser amplifier, arefacilitated.

[0113] In the case of FIG. 4, the first square rod 21, the second squarerod 22, the 90-degree polarization rotator 25, the third square rod 23,and the fourth square rod 24 are arranged in this order. However, thisembodiment 1 is not limited to this configuration. As an alternative,the pumping module can have either of the following configurations (A)to (C), and the same advantages are provided in either case.

[0114] (A) The second square rod 22, the first square rod 21, the90-degree polarization rotator 25, the third square rod 23, and thefourth square rod 24 are arranged in this order.

[0115] (B) The first square rod 21, the second square rod 22, the90-degree polarization rotator 25, the fourth square rod 24, and thethird square rod 23 are arranged in this order.

[0116] (C) The second square rod 22, the first square rod 21, the90-degree polarization rotator 25, the fourth square rod 24, and thirdsquare rod 23 are arranged in this order.

[0117] In other words, when defining the 90-degree polarization rotator25 as a boundary, the plurality of square rods can be classified intothe first square rod group that consists of the first and second squarerods 21 and 22 and the second square rod group that consists of thethird and fourth square rods 23 and 24, and the order of the first andsecond square rods 21 and 22 in the first square rod group and the orderof the third and fourth square rods 23 and 24 in the second square rodgroup can be arbitrarily determined.

[0118] The pumping module is provided with one set of the first squarerod 21, the second square rod 22, the 90-degree polarization rotator 25,the third square rod 23, and the fourth square rod 24. As analternative, the pumping module can have m sets of those components (mis a natural number) that are so constructed that their optical axescoincide with one another (i.e., they shares the optical axis 26) andare cascaded.

[0119] Even in this case, the pumping module can prevent any decrease inthe extinction ratio regardless of a variation in the orientation of thebirefringence axes of each of the plurality of square rods and can alsoprevent any decrease in the energy efficiency of a laser apparatus towhich the pumping module is applied and any decrease in the beamquality. In addition, because the incident laser light passes through athermal lens with respect to the direction of the x axis, having athermal lens effect with respect to a direction perpendicular to thedirection of heat sinking 2 m times, and also passes through a thermallens with respect to the direction of the y axis, having another thermallens effect with respect to a direction perpendicular to the directionof heat sinking 2 m times, there causes no difference between thethermal lens with respect to the direction of the x axis and the otherthermal lens with respect to the direction of the y axis and theastigmatism can be compensated for.

[0120] Furthermore, because the cascade connection increases the numberof square rods, the total gain to be given to the laser light isincreased and the efficiency of such a laser apparatus to which thepumping module is applied, as a laser oscillator or a laser amplifier,is improved.

[0121] As mentioned above, in accordance with this embodiment 1, thepumping module is provided with a first square rod group including afirst square rod 21 having an optical axis 26 and having a couple ofheat sinking surfaces normal to a direction of y axis perpendicular tothe optical axis 26 and a second square rod 22 having the optical axis26 in common with the first square rod and having a couple of heatsinking surfaces normal to a direction of x axis perpendicular to theoptical axis 26 and the direction of the y axis, a second square rodgroup including a third square rod 23 having the optical axis 26 incommon with the first square rod and having a couple of heat sinkingsurfaces normal to the direction of the y axis and a fourth square rod24 having the optical axis 26 in common with the first square rod andhaving a couple of heat sinking surfaces normal to the direction of thex axis, and a 90-degree polarization rotator 25 disposed between thefirst and second square rod groups and having the optical axis 26 incommon with the first square rod, for rotating a polarization of laserlight passing therethrough by 90 degrees. As a result, the presentinvention offers an advantage of being able to prevent any decrease inthe extinction ratio regardless of a variation in the orientation of thebirefringence axes of each of the plurality of square rods, and toprevent the occurrence of a difference between a thermal lens withrespect to the direction of the x axis and another thermal lens withrespect to the direction of the y axis, thereby preventing theoccurrence of astigmatism.

[0122] Furthermore, in accordance with this embodiment 1, the firstsquare rod group is provided with the first square rod 21 and the secondsquare rod 22 in each of which the laser light propagates along azig-zag optical path between the couple of heat sinking surfacesthereof, and the second square rod group is provided with the thirdsquare rod 23 and the fourth square rod 24 in each of which the laserlight propagates along a zig-zag optical path between the couple of heatsinking surfaces thereof. As a result, the present embodiment offersanother advantage of being able to make the temperature gradients causedin the direction of heat sinking uniform in each of the plurality ofsquare rods, thereby preventing thermal lens effects from beingproduced.

[0123] In addition, in accordance with this embodiment 1, a plurality ofpumping modules can be provided so that they have an optical axis incommon with one another and are cascaded. As a result, the increasednumber of square rods can increase the gain to be given to the laserlight.

[0124] Furthermore, in accordance with this embodiment 1, there isprovided a laser oscillator including a pumping module according toembodiment 1, a total reflection mirror that is perpendicular to anoptical axis 26 of the pumping module, and a partial reflection mirrorthat is disposed so that the pumping module is sandwiched between thepartial reflection mirror and the total reflection mirror and that isperpendicular to the optical axis 26. As a result, the presentembodiment offers another advantage of being able to prevent anydecrease in the efficiency of energy and any decrease in the beamquality without having to use a mechanism of compensating forastigmatism.

[0125] In addition, in accordance with this embodiment 1, the pumpingmodule can amplify and output an input laser light. As a result, thepresent embodiment offers another advantage of being able to provide alaser amplifier that can prevent any decrease in the efficiency ofenergy and any decrease in the beam quality without having to use amechanism of compensating for astigmatism.

[0126] Embodiment 2.

[0127]FIG. 7 is a diagram showing the structure of a pumping moduleaccording to embodiment 2 of the present invention.

[0128] In FIG. 7, reference numeral 27 denotes a fifth square rod,reference numeral 28 denotes a sixth square rod, and reference numerals25 and 26, which are also shown in FIG. 1, denote a 90-degreepolarization rotator and an optical axis, respectively. The fifth andsixth square rods 27 and 28 and the 90-degree polarization rotator 25have the optical axis 26 in common.

[0129] In the fifth square rod 27, heat generated in a half of thereofis dissipated in a direction of y axis and heat generated in theremaining half thereof is dissipated in a direction of x axis, and inthe sixth square rod 28, heat generated in a half of thereof isdissipated in the direction of the x axis and heat generated in theremaining half thereof is dissipated in the direction of the y axis.

[0130] In other words, it can be considered that the fifth square rod 27is equivalent to the one in which the first and second square rods 21and 22 in the first square rod group in accordance with embodiment 1 areintegrally formed. Similarly, the sixth square rod 28 is equivalent tothe one in which the third and fourth square rods 23 and 24 in thesecond square rod group in accordance with embodiment 1 are integrallyformed.

[0131] The 90-degree polarization rotator 25 is placed between the fifthand sixth square rods 27 and 28. The fifth square rod 27 and the sixthsquare rod 28 are pumped in much the same way, and temperature gradientsand birefringence are caused in much the same way in the fifth and sixthsquare rods 27 and 28.

[0132] In the case of the pumping module as shown in FIG. 7 which isconstructed as mentioned above, the Jones matrix is also given by theequation (6). Therefore, the pumping module of FIG. 7 can also offer anadvantage of being able to prevent any decrease in the extinction ratioregardless of a variation in the orientation of the birefringence axesof each of the plurality of square rods. Application of the pumpingmodule to a laser apparatus, such as a laser oscillator or a laseramplifier, makes it possible to prevent any decrease in the efficiencyof energy and any decrease in the beam quality.

[0133] Furthermore, the pumping module of this embodiment providessimilar thermal lens effects, and can prevent the occurrence of adifference between a thermal lens with respect to the direction of the xaxis and another thermal lens with respect to the direction of the yaxis, thereby preventing the occurrence of astigmatism. Therefore, alaser apparatus to which the pumping module is applied such as a laseroscillator or a laser amplifier need not use a mechanism of compensatingfor astigmatism and the optical system can be simplified.

[0134] Particularly, as compared with embodiment 1, because the numberof square rods in the pumping module of this embodiment 2 is reduced,the alignment of each of the plurality of square rods is facilitated andprocesses such as grinding and coating of each of the plurality ofsquare rods can be omitted, and therefore the cost of the pumping modulecan be reduced.

[0135] As mentioned above, in accordance with this embodiment 2, thefirst square rod group is provided with a fifth square rod 27 in which afirst square rod 21 and a second square rod 22 are integrally formed,and the second square rod group is provided with a sixth square rod 28in which a third square rod 23 and a fourth square rod 24 are integrallyformed. As a result, the present embodiment offers an advantage of beingable to facilitate the alignment of each of the plurality of square rodsand to omit processes such as grinding and coating of each of theplurality of square rods, thereby reducing the cost of the pumpingmodule.

[0136] Embodiment 3.

[0137]FIG. 8 is a diagram showing the structure of a pumping moduleaccording to embodiment 3 of the present invention.

[0138] In FIG. 8, reference numeral 31 denotes a seventh square rod,reference numeral 32 denotes an eighth square rod, reference numeral 33denotes a ninth square rod, reference numeral 34 denotes a tenth squarerod, reference numeral 35 denotes a first 90-degree polarizationrotator, and reference numeral 36 denotes a second 90-degreepolarization rotator. Reference numeral 26, which is also shown in FIG.1, denotes an optical axis. The seventh through tenth square rods 31 to34 and the first and second 90-degree polarization rotators 35 and 36have the optical axis 26 in common.

[0139] Both a couple of heat sinking surfaces of the seventh square rod31 and a couple of heat sinking surfaces of the eighth square rod 32 arenormal to a direction of y axis and both a couple of heat sinkingsurface of the ninth square rod 33 and a couple of heat sinking surfaceof the tenth square rod 34 are normal to a direction of x axis. Thefirst 90-degree polarization rotator 35 is placed between the seventhsquare rod 31 and the eighth square rod 32, and the second 90-degreepolarization rotator 36 is placed between the ninth square rod 33 andthe tenth square rod 34.

[0140] The seventh square rod 31 and the eighth square rod 32 are pumpedin much the same way, and temperature gradients and birefringence arecaused in much the same way in the seventh and eighth square rods 31 and32. Furthermore, the ninth square rod 33 and the tenth square rod 34 arepumped in much the same way, and temperature gradients and birefringenceare caused in much the same way in the ninth and tenth square rods 33and 34. The second 90-degree polarization rotator 36 rotates thepolarization of laser light incident thereupon by 90 degrees (i.e., −90degrees) opposite in direction to the polarization rotation done by thefirst 90-degree polarization rotator 35.

[0141] Next, a description will be made as to the operation of thepumping module according to embodiment 3 of the present invention.

[0142] Laser light travels on the optical axis 26, passes through theseventh square rod 31, and is then incident upon to the first 90-degreepolarization rotator 35. The first 90-degree polarization rotator 35emits the incident laser light towards the eighth square rod 32 afterrotating the polarization of the incident laser light by 90 degrees. Thelaser light emitted out of the first 90-degree polarization rotator 35further passes through the eighth square rod 32.

[0143] The laser light emitted out of the eighth square rod 33 travelson the optical axis 26, passes through the ninth square rod 33, and isthen incident upon the second 90-degree polarization rotator 36. Thesecond 90-degree polarization rotator 36 emits the incident laser lighttowards the tenth square rod 34 after rotating the polarization of theincident laser light by −90 degrees. The laser light emitted out of thesecond 90-degree polarization rotator 36 further passes through thetenth square rod 34.

[0144] Pump light emitted out of a pump light source not shown in thefigure is absorbed by the seventh through tenth square rods 31 to 34 anda gain is produced, and the laser light traveling in the direction ofthe optical axis 26 is amplified by the seventh through tenth squarerods 31 to 34.

[0145] The Jones matrix J_(R31-R32) of the optical system that consistsof the seventh square rod 31, the first 90-degree polarization rotator35, and the eighth square rod 32 is given by the following equation (7).$\begin{matrix}\begin{matrix}{J_{{R\quad 31} - {R\quad 32}} = {J_{R\quad 32} \cdot J_{Rot} \cdot J_{R\quad 31}}} \\{= {{J_{B}( {{\frac{\pi}{2} + \theta_{1}},\delta_{1}} )} \cdot \begin{pmatrix}0 & {- 1} \\1 & 0\end{pmatrix} \cdot {J_{B}( {{\frac{\pi}{2} + \theta_{1}},\delta_{1}} )}}} \\{= \begin{pmatrix}0 & {- 1} \\1 & 0\end{pmatrix}}\end{matrix} & {{Equation}\quad (7)}\end{matrix}$

[0146] As can be seen from the equation (7), the first polarizationrotating optical system that consists of the seventh square rod 31, thefirst 90-degree polarization rotator 35, and the eighth square rod 32functions as a 90-degree polarization rotator despite the bearing angleθ₁ and the phase difference δ₁, and rotates the polarization of laserlight incident thereupon by 90 degrees.

[0147] Similarly, the Jones matrix J_(R33-R34) of the optical systemthat consists of the ninth square rod 33, the second 90-degreepolarization rotator 36, and the tenth square rod 34 is given by thefollowing equation (8). $\begin{matrix}\begin{matrix}{J_{{R\quad 33} - {R\quad 34}} = {J_{R\quad 34} \cdot J_{Rot} \cdot J_{R\quad 33}}} \\{= {{J_{B}( {{\pi - \theta_{2}},\delta_{2}} )} \cdot \begin{pmatrix}0 & 1 \\{- 1} & 0\end{pmatrix} \cdot {J_{B}( {{\pi - \theta_{2}},\delta_{2}} )}}} \\{= \begin{pmatrix}0 & 1 \\{- 1} & 0\end{pmatrix}}\end{matrix} & {{Equation}\quad (8)}\end{matrix}$

[0148] In other words, the equation (8) shows that the secondpolarization rotating optical system that consists of the ninth squarerod 33, the second 90-degree polarization rotator 36, and the tenthsquare rod 34 functions as a 90-degree polarization rotator despite thebearing angle θ₂ and the phase difference δ₂, and rotates thepolarization of laser light incident thereupon by −90 degrees.

[0149] Therefore, the Jones matrix J_(All) of the pumping module asshown in FIG. 8 is given by the following equation (9). $\begin{matrix}\begin{matrix}{J_{A\quad l\quad l} = {J_{R\quad 34} \cdot J_{Rot} \cdot J_{R\quad 33} \cdot J_{R\quad 32} \cdot J_{Rot} \cdot J_{R\quad 31}}} \\{= {\begin{pmatrix}0 & {- 1} \\1 & 0\end{pmatrix} \cdot \begin{pmatrix}0 & 1 \\{- 1} & 0\end{pmatrix}}} \\{= \begin{pmatrix}1 & 0 \\0 & 1\end{pmatrix}}\end{matrix} & {{Equation}\quad (9)}\end{matrix}$

[0150] Because the matrix of the equation (9) doesn't depend on thebearing angles θ₁ and θ₂ and the phase differences δ₁ and δ₂, and is aunit matrix having 2 rows and 2 columns, it can be understood that thepumping module doesn't change the polarization of the incident laserlight. Therefore, because the pumping module doesn't change thepolarization of the incident laser light regardless of a variation inthe orientation of the birefringence axes of each of the plurality ofsquare rods, no decrease occurs in the extinction ratio and any decreasein the efficiency of energy and any decrease in the beam quality can beprevented.

[0151] In this embodiment, the second 90-degree polarization rotator 36rotates the polarization of laser light incident thereupon by 90 degrees(i.e., −90 degrees) opposite in direction to the rotation (+90 degrees)done by the first 90-degree polarization rotator 35. This embodiment 3is not limited to this case. As an alternative, the second 90-degreepolarization rotator 36 can rotate the polarization of laser lightincident thereupon by 90 degrees in the same direction as the first90-degree polarization rotator 35.

[0152] In this case, the pumping module as shown in FIG. 8 functions asa 180-degree polarization rotator (having a unit matrix whosecoefficient is −1), and because the pumping module doesn't change thepolarization of the incident laser light regardless of a variation inthe orientation of the birefringence axes of each of the plurality ofsquare rods, no decrease occurs in the extinction ratio and any decreasein the efficiency of energy and any decrease in the beam quality can beprevented.

[0153] Because, as for the direction of the x axis, the incident laserlight passes through a thermal lens having a thermal lens effect withrespect to the direction of heat sinking twice and passes through athermal lens having a thermal lens effect with respect to a directionperpendicular to the direction of heat sinking twice, and, as for thedirection of the y axis, the incident laser light also passes through athermal lens having a thermal lens effect with respect to the directionof heat sinking twice and passes through a thermal lens having a thermallens effect with respect to a direction perpendicular to the directionof heat sinking twice, there causes no difference between the thermallenses with respect to the direction of the x axis and the other thermallenses with respect to the direction of the y axis and the astigmatismcan be therefore compensated for. Therefore, a laser apparatus to whichthe pumping module is applied such as a laser oscillator or a laseramplifier need not use a mechanism of compensating for astigmatism andthe optical system can be simplified.

[0154] In the above explanation, it is assumed that the laser lightpassing through the seventh through tenth square rods 31 to 34propagates in parallel with the optical axis 26. This embodiment 3 isnot limited to this configuration and each of the seventh through tenthsquare rods 31 to 34 can be formed, as previously mentioned inEmbodiment 1, so that the laser light propagates along a zigzag opticalpath which is bent several times in the direction of the y axis in theseventh square rod 31, along a zigzag optical path which is bent severaltimes in the direction of the y axis in the eighth square rod 32, alonga zigzag optical path which is bent several times in the direction ofthe x axis in the ninth square rod 33, and along a zigzag optical pathwhich is bent several times in the direction of the x axis in the tenthsquare rod 34.

[0155] This configuration does not cause any thermal lens effect withrespect to the direction of heat sinking in each of the plurality ofsquare rods in each of which the incident laser light propagates along azigzag optical path because the temperature gradients caused in thedirection of heat sinking are made uniform. Therefore, because, as forthe direction of the x axis, the incident laser light passes through athermal lens having a thermal lens effect with respect to a directionperpendicular to the direction of heat sinking twice, and, as for thedirection of the y axis, also passes through a thermal lens having athermal lens effect with respect to a direction perpendicular to thedirection of heat sinking twice, there causes no difference between thethermal lens with respect to the direction of the x axis and the otherthermal lens with respect to the direction of the y axis and theastigmatism can be compensated for.

[0156] In addition, the thermal lens of the entire pumping modulebecomes small because it doesn't receive the influence of the thermallens effect with respect to the direction of heat sinking, and thedesign and structure of such a laser apparatus to which the pumpingmodule is applied to, as a laser oscillator or a laser amplifier, arefacilitated.

[0157] The pumping module is provided with one first polarizationrotating optical system including the seventh square rod 31, the first90-degree polarization rotator 35 and the eighth square rod 32, and onesecond polarization rotating optical system including the ninth squarerod 33, the second 90-degree polarization rotator 36 and the tenthsquare rod 34. As an alternative, the pumping module can have m firstpolarization rotating optical systems and m second polarization rotatingoptical systems (m is a natural number) that are so constructed thatthey are cascaded.

[0158] Because this configuration makes it possible not to change thepolarization of incident laser light regardless of a variation in theorientation of the birefringence axes of each of the plurality of squarerods, the pumping module can prevent any decrease in the extinctionratio and can also prevent any decrease in the energy efficiency of alaser apparatus to which the pumping module is applied and any decreasein the beam quality. In addition, because, as for the direction of the xaxis, the incident laser light passes through a thermal lens having athermal lens effect with respect to a direction perpendicular to thedirection of heat sinking 2 m times, and, as for the direction of the xaxis, also passes through a thermal lens having another thermal lenseffect with respect to a direction perpendicular to the direction ofheat sinking 2 m times, there causes no difference between the thermallens with respect to the direction of the x axis and the other thermallens with respect to the direction of the y axis and the astigmatism canbe compensated for.

[0159] Furthermore, because the cascade connection increases the numberof square rods, the total gain to be given to the laser light isincreased and the efficiency of such a laser apparatus to which thepumping module is applied, as a laser oscillator or a laser amplifier,is improved.

[0160] As mentioned above, in accordance with this embodiment 3, thepumping module is provided with a first polarization rotating opticalsystem including seventh and eighth square rods 31 and 32 having anoptical axis 26 and each having a couple of heat sinking surfaces whichare normal to a direction of y axis perpendicular to the optical axis26, and a first 90-degree polarization rotator 35 having the opticalaxis 26 in common with the seventh and eighth square rods 31 and 32 anddisposed between the seventh and eighth square rods 31 and 32, forrotating the polarization of laser light passing therethrough by 90degrees; and a second polarization rotating optical system includingninth and tenth square rods 33 and 34 having the optical axis 26 incommon with the seventh and eighth square rods 31 and 32 and each havinga couple of heat sinking surfaces which are normal to a direction of xaxis perpendicular to the optical axis 26 and the direction of the yaxis, and a second 90-degree polarization rotator 36 having the opticalaxis 26 in common with the seventh and eighth square rods 31 and 32 anddisposed between the ninth and tenth square rods 33 and 34, for rotatingthe polarization of laser light passing therethrough by 90 degrees. As aresult, the present embodiment offers an advantage of being able toprevent any decrease in the extinction ratio regardless of a variationin the orientation of the birefringence axes of each of the plurality ofsquare rods, and to prevent the occurrence of a difference between athermal lens with respect to the direction of the x axis and anotherthermal lens with respect to the direction of the y axis, therebypreventing the occurrence of astigmatism.

[0161] Furthermore, in accordance with this embodiment 3, the firstpolarization rotating optical system is provided with the seventh squarerod 31 and the eighth square rod 32 in each of which the laser light isallowed to propagate along a zig-zag optical path between the couple ofheat sinking surfaces thereof, and the second polarization rotatingoptical system is provided with the ninth square rod 33 and the tenthsquare rod 34 in each of which the laser light is allowed to propagatealong a zig-zag optical path between the couple of heat sinking surfacesthereof. As a result, the present embodiment offers another advantage ofbeing able to make the temperature gradients caused in the direction ofheat sinking uniform in each of the plurality of square rods, therebypreventing thermal lens effects from being produced.

[0162] In addition, in accordance with this embodiment 3, a plurality ofpumping modules can be provided so that their optical axes coincide withone another and they are cascaded. As a result, the increased number ofsquare rods can increase the gain to be given to the laser light.

[0163] Embodiment 4.

[0164] In either of embodiments 1 to 3, the pumping module having atransmission optical system that uses one or more 90-degree polarizationrotators is explained. In contrast, in accordance with this embodiment4, a pumping module having a reflection optical system that uses a45-degree polarization rotator is provided.

[0165]FIG. 9 is a diagram showing the structure of the pumping moduleaccording to embodiment 4 of the present invention.

[0166] In FIG. 9, reference numeral 41 denotes an eleventh square rod,reference numeral 42 denotes a twelfth square rod, reference numeral 43denotes a first 45-degree polarization rotator, reference numeral 44denotes a first total reflection mirror, and reference numeral 26, whichis also shown in FIG. 1, denotes an optical axis of the pumping moduleaccording to this embodiment 4. The eleventh and twelfth square rods 41and 42 and the first 45-degree polarization rotator 43 have the opticalaxis 26 in common. The first total reflection mirror 44 has a reflectionsurface normal to the optical axis 26. The eleventh square rod 41 has acouple of heat sinking surfaces normal to a direction of y axis, and thetwelfth square rod 42 has a couple of heat sinking surfaces normal to adirection of x axis. The first 45-degree polarization rotator 43 isplaced between the twelfth square rod 42 and the first total reflectionmirror 44.

[0167] Next, a description will be made as to the operation of thepumping module of embodiment 4 of the present invention.

[0168] Laser light that travels on the optical axis 26 and is incidentupon the pumping module passes through the eleventh square rod 41 andthe twelfth square rod 42 in succession. The laser light is thenincident upon the first 45-degree polarization rotator 43. The first45-degree polarization rotator 43 rotates the polarization of theforward traveling laser light incident thereupon from the twelfth squarerod 42 only by 45 degrees, and emits it towards the first totalreflection mirror 44.

[0169] The first total reflection mirror 44 reflects the laser light soas to make it incident upon the first 45-degree polarization rotator 43.The first 45-degree polarization rotator 43 further rotates thepolarization of the backward traveling laser light incident thereuponfrom the first total reflection mirror 44 only by 45 degrees, and emitsit towards the twelfth square rod 42. The laser light emitted out of thefirst 45-degree polarization rotator 43 passes through the twelfth 42square rod and the eleventh square rod 41 in order opposite to the orderin which the laser light has passed through them while travelingforwards in the pumping module, and then emerges from the pumpingmodule. Pump light emitted out of a pump light source not shown in thefigure is absorbed by the eleventh and twelfth square rods 41 and 42,and a gain is produced, and the laser light traveling forwards andbackwards along the optical axis 26 is thus amplified by the eleventhand twelfth square rods 41 and 42.

[0170] The pumping module of FIG. 9 will be explained by using a Jonesmatrix equation, after the manner as mentioned in embodiment 1.

[0171] In FIG. 9, it is assumed that the reflectivity of the first totalreflection mirror 44 is M, and the Jones matrix of the eleventh squarerod 41 is J_(R41) and the Jones matrix of the twelfth square rod 43 isJ_(R42). Each of the Jones matrices J_(R41) and J_(R42) is similar tothat as shown in the equation (2) described in Embodiment 1.

[0172] The Jones matrix J_(Rot)′ of the first 45-degree polarizationrotator 43 is given by the following equation (10). Because the Jonesmatrix of the polarization rotator is a rotation matrix, the followingrelationship: J_(Rot)′·J_(Rot)′=J_(Rot) is established. In other words,when laser light travels forwards and backwards in the pumping module soas to pass through the first 45-degree polarization rotator 43 twice,the first 45-degree polarization serves as a 90-degree polarizationrotator as shown in either of Embodiments 1 to 3. $\begin{matrix}{J_{Rot}^{\prime} = \begin{pmatrix}{\cos \frac{\pi}{4}} & {{- \sin}\frac{\pi}{4}} \\{\sin \frac{\pi}{4}} & {\cos \frac{\pi}{4}}\end{pmatrix}} & {{Equation}\quad (10)}\end{matrix}$

[0173] When taking the order in which the laser light passes through allthe components of the pumping module and the above-mentioned Jonesmatrices into consideration, the Jones matrix J_(All) of the entirepumping module of FIG. 9 is calculated by using the following equation(11). $\begin{matrix}\begin{matrix}{J_{A\quad l\quad l} = {J_{R\quad 41} \cdot J_{R\quad 42} \cdot J_{Rot}^{\prime} \cdot M \cdot J_{Rot}^{\prime} \cdot J_{R\quad 42} \cdot J_{R\quad 41}}} \\{= {M \cdot J_{R\quad 41} \cdot J_{R\quad 42} \cdot J_{Rot}^{\prime} \cdot J_{Rot}^{\prime} \cdot J_{R\quad 42} \cdot J_{R\quad 41}}} \\{= {M \cdot J_{R\quad 41} \cdot J_{R\quad 42} \cdot J_{Rot} \cdot J_{R\quad 42} \cdot J_{R\quad 41}}} \\{= {M\begin{pmatrix}0 & {- 1} \\1 & 0\end{pmatrix}}}\end{matrix} & {{Equation}\quad (11)}\end{matrix}$

[0174] As can be seen from the equation (11), even in the case of thepumping module having a reflection optical system as shown in FIG. 9,the pumping module operates in such a manner that the outgoing polarizedlight vector (E_(Out), E_(yout))^(T) must be perpendicular to theincoming polarized light vector (E_(x), E_(y))^(T) given by the equation(1), like that of embodiment 1.

[0175] In general, when the 90-degree rotation matrix J_(Rot)(=J_(Rot)′·J_(Rot)′) given by the equation (3) is multiplied by anarbitrary matrix Q having 2 rows and 2 columns from the left sidethereof, and is also multiplied by the transpose QT of the matrix Q fromthe right side thereof, the following equation:Q·J_(Rot)·Q^(T)=|Q|·J_(Rot) is obtained.

[0176] The result of the above matrix multiplication shows that theresult is equal to the 90-degree rotation matrix J_(rot) itselfmultiplied by a number and the actions of the arbitrary matrix Q and thetranspose of the matrix Q on the 90-degree rotation matrix J_(Rot)result in that only the determinant of the matrix Q appears as thecoefficient of the 90-degree rotation matrix J_(Rot). Because thecoefficient |Q| doesn't have an essential influence on the arbitrarytwo-dimension vector (especially, because the Jones matrix is aso-called unitary matrix, the absolute value of the determinant of thematrix becomes 1), only the 90-degree rotation matrix J_(Rot) is appliedto the two-dimension vector while the two-dimension vector is notinfluenced by the matrix Q.

[0177] In accordance with this embodiment 4, because the pumping modulehas the reflection optical system based on the above-mentioned idea andthe plurality of optical components act on the polarization of the laserlight traveling backward in the pumping module in order opposite to thatin which they act on the laser light traveling forward in the pumpingmodule, the above matrix multiplication is implemented optically.

[0178] Furthermore, thermal lens effects are compensated for.

[0179] In other words, the thermal lens effect with respect to thedirection of the x axis and the thermal lens effect with respect to thedirection of the y axis are counterbalanced and no astigmatism occursbecause the laser light passes through the eleventh square rod 41, inwhich heat is dissipated in the direction of the y axis, twice in theforward and backward directions, and also passes through the twelfthsquare rod 42, in which heat is dissipated in the direction of the zaxis, twice in the forward and backward directions.

[0180] Thus, the pumping module having the reflection optical system ofFIG. 9 provides the same results as those provided by that of embodiment1.

[0181] Particularly, because the pumping module of FIG. 9 consists ofthe reflection optical system, the same pumping need not be performed onthe plurality of different square rods as long as the Jones matrixJ_(R41)·J_(R42) for the laser light traveling backwards doesn't changefaster than the light speed with respect to the Jones matrixJ_(R42)·J_(R41) for the laser light traveling forwards (i.e., as long asno difference occurs between both the Jones matrices) while the lossincreases due to the reflectivity M of the first total reflection mirror44 and so on.

[0182] An example of the structure of such a laser apparatus, to whichthe pumping module of FIG. 9 is applied, as a laser oscillator or alaser amplifier, will be explained.

[0183] <Laser oscillator 1>

[0184]FIG. 10 is a diagram showing the structure of a laser oscillatorto which the pumping module according to embodiment 4 of the presentinvention is applied. The same reference numerals as shown in FIG. 9denote the same components or like components.

[0185] In FIG. 10, reference numeral 45 denotes a partial reflectionmirror that is placed on an optical axis, 26 together with the first45-degree polarization rotator 43 and the first total reflection mirror44 so that the eleventh and twelfth square rods 41 and 42 are sandwichedbetween the first 45-degree polarization rotator 43 and the partialreflection mirror 45. The partial reflection mirror 45 has a reflectionsurface normal to the optical axis 26. The space between the partialreflection mirror 45 and the first total reflection mirror 44 is acavity. A arrow designated by reference character a shows a forward pathof the laser light incident upon the pumping module of the laseramplifier, and another arrow designated by reference character b shows abackward path of the laser light that emerges from the pumping module ofthe laser amplifier.

[0186] Next, a description will be made as to the operation of the laseroscillator.

[0187] In a case where the pumping module is used as a laser oscillator,laser oscillation is caused when the polarization and phase of the laserlight incident upon the pumping module coincide with those of the laserlight that has traveled between the total reflection mirror and thepartial reflection mirror in the pumping module an even number of times.By adjusting the cavity length of the laser oscillator, the phase of thelaser light incident upon the pumping module can be made to coincidewith that of the laser light that has traveled between the totalreflection mirror and the partial reflection mirror in the pumpingmodule an even number of times. Therefore, a description will be made asto the coincidence between the polarization of the laser light incidentupon the pumping module can be made to coincide with that of the laserlight that has traveled between the total reflection mirror and thepartial reflection mirror in the pumping module an even number of times.

[0188] As shown in FIG. 10, when the laser light passing through thepartial reflection mirror 45 along the forward path a is linearlypolarized in the direction of the y axis, for example, as described inthe explanation of the operation of the pumping module of FIG. 9, afterthe laser light initially traveling along the forward path a hastraveled between the partial reflection mirror and the total reflectionmirror for the first time, the laser light is linearly polarized in thedirection of the x axis. The laser light thus linearly polarized in thedirection of the x axis and traveling along the backward path b is thenreflected by the partial reflection mirror 45 and travels along theforward direction a while being linearly polarized in the direction ofthe x axis. After that, when the laser light traveling along the forwardpath a travels between the partial reflection mirror and the totalreflection mirror for the second time, it is linearly polarized in thedirection of the y axis.

[0189] Therefore, because the polarization and phase of the laser lightincident upon the pumping module coincide with those of the laser lightthat has traveled between the total reflection mirror and the partialreflection mirror in the pumping module (2 multiplied by a naturalnumber) times, i.e., an even number of times, laser oscillation isgenerated within the cavity between the partial reflection mirror 45 andthe first total reflection mirror 44. The laser light thus caused by thelaser oscillation emerges from the partial reflection mirror 45.

[0190] The above-mentioned operations and polarization statuses aresummarized as follows: traveling along the forward path a (linearlypolarized in the direction of the y axis)→further traveling forwards andthen backwards in the pumping module→traveling along the backward path b(linearly polarized in the direction of the x axis)→reflected by thepartial reflection mirror 45→traveling along the forward path a(linearly polarized in the direction of the x axis)→further travelingforwards and then backwards in the pumping module→traveling along thebackward path b (linearly polarized in the direction of the yaxis)→reflected by the partial reflection mirror 45→traveling along theforward path a (linearly polarized in the direction of the y axis)→ . ..

[0191] <Laser oscillator 2>

[0192] An addition of a second 45-degree polarization rotator to thestructure of FIG. 10 makes it possible for the laser oscillator togenerate laser light with one round trip.

[0193]FIG. 11 is a diagram showing the structure of such a laseroscillator to which the pumping module according to embodiment 4 of thepresent invention is applied. In the figure, the same reference numeralsas shown in FIGS. 1, 9, and 10 denote the same components or likecomponents.

[0194] In FIG. 11, reference numeral 46 denotes a second 45-degreepolarization rotator placed on an optical axis 26 and between a partialreflection mirror 45 and an eleventh square rod 41.

[0195] Next, a description will be made as to the operation of the laseroscillator.

[0196] Laser light linearly polarized in a direction of y axis travelsalong a forward path a, and is then reflected by a total reflectionmirror and travels backwards. The laser light traveling along a backwardpath b is thus linearly polarized in a direction of x axis. Thepolarization of the laser light traveling along the backward path b isfurther rotated only by 45 degrees with respect to the direction of thex axis by the second 45-degree polarization rotator 46. The polarizationof the laser light is further rotated only by 45 degrees with respect tothe direction of the x axis by the second 45-degree polarization rotator46 after reflected by the partial reflection mirror 45.

[0197] Therefore, after the laser light traveling along the backwardpath b and linearly polarized in the direction of the x axis has passedthrough the second 45-degree polarization rotator 46 twice, thepolarization of the laser light is finally rotated by 90 degrees withrespect to the direction of the x axis and therefore the laser light islinearly polarized in the direction of the y axis when traveling alongthe forward path a for the second time. Thus, in the laser oscillator ofFIG. 11, the polarization and phase of the laser light incident upon thepumping module coincide with those of the laser light that has traveledbetween the total reflection mirror and the partial reflection mirror inthe pumping module only once, and laser oscillation is generated withinthe cavity between the first total reflection mirror 44 and the partialreflection mirror 45. The laser light thus caused by the laseroscillation emerges from the partial reflection mirror 45.

[0198] The above-mentioned operation of the laser oscillator issummarized as follows: traveling along the forward path a (linearlypolarized in the direction of the y axis)→further traveling forwards andthen backwards in the pumping module→traveling along the backward path b(linearly polarized in the direction of the x axis)→passing through thesecond 45-degree polarization rotator 46→reflected by the partialreflection mirror 45→passing through the second 45-degree polarizationrotator 46→traveling along the forward path a (linearly polarized in thedirection of the y axis)→ . . .

[0199] <Laser oscillator 3>

[0200]FIG. 12 is a diagram showing the structure of a laser oscillatorto which the pumping module according to embodiment 4 of the presentinvention is applied. In the figure, the same reference numerals asshown in FIGS. 1, 9, and 10 denote the same components or likecomponents.

[0201] In FIG. 12, reference numeral 47 denotes a polarizer throughwhich laser light linearly polarized in a direction of x axis can pass,the polarizer being placed on an optical axis 26 and between a partialreflection mirror 45 and an eleventh square rod 41, and referencenumeral 48 denotes a second total reflection mirror for totallyreflecting laser light reflected by the polarizer 47.

[0202] The polarizer 47 allows incident laser light (laser light of apredetermined polarization) linearly polarized in the direction of the xaxis to pass therethrough while making the emerging direction of thelaser light coincide with the incident direction, and reflects incidentlaser light linearly polarized in the direction of the y axis (laserlight of a polarization perpendicular to the predetermined polarization)in a direction perpendicular to the optical axis 26. The second totalreflection mirror 48 is disposed in this direction perpendicular to theoptical axis 26, and can reflect the laser light reflected by thepolarizer 47 towards the polarizer 47.

[0203] Next, a description will be made as to the operation of the laseroscillator.

[0204] Laser light linearly polarized in the direction of the y axistravels along a forward path a, and is then reflected by the first totalreflection mirror and travels backwards. The laser light traveling alonga backward path b is thus linearly polarized in the direction of the xaxis. Because the laser light traveling along the backward path b islinearly polarized in the direction of the x axis, it passes through thepolarizer 47 and is then reflected by the partial reflection mirror 45.After that, the laser light passes through the polarizer 47 again andreaches the forward path a. At this time, because the laser lighttraveling along the forward path a is linearly polarized in thedirection of the x axis, the laser light becomes the one linearlypolarized in the direction of the y axis while traveling along thebackward path b after reflected by the first total reflection mirror andtraveling backwards for the second time.

[0205] Because the laser light traveling along the backward path b islinearly polarized in the direction of the y axis, it is reflected in adirection perpendicular to the optical axis 26 by the polarizer 47 sothat it is directed towards the second total reflection mirror 48, andis then reflected towards the polarizer 47 by the total reflectionmirror 48 and is further reflected by the polarizer 47. The laser lightis finally linearly polarized in the direction of the y axis and travelsalong the forward path a.

[0206] Thus, in the laser oscillator of FIG. 12, the polarization andphase of the laser light incident upon the pumping module coincide withthose of the laser light that has traveled between the first totalreflection mirror and the partial reflection mirror in the pumpingmodule twice, and laser oscillation can be generated within the cavitybetween the first total reflection mirror 44 and the partial reflectionmirror 45. The laser light thus caused by the laser oscillation emergesfrom the partial reflection mirror 45.

[0207] The above-mentioned operation of the laser oscillator issummarized as follows: traveling along the forward path a (linearlypolarized in the direction of the y axis)→further traveling forwards andthen backwards in the pumping module→traveling along the backward path b(linearly polarized in the direction of the x axis)→passing through thepolarizer 47→reflected by the partial reflection mirror 45→passingthrough the polarizer 47→traveling along the forward path a (linearlypolarized in the direction of the x axis)→further traveling forwards andthen backwards in the pumping module→traveling along the backward path b(linearly polarized in the direction of the y axis)→reflected by thepolarizer 47→reflected by the second total reflection mirror48→reflected by the polarizer 47→traveling along the forward path a(linearly polarized in the direction of the y axis)→ . . .

[0208] <Laser amplifier>

[0209] Next, a laser amplifier that uses the pumping module of FIG. 9will be explained.

[0210]FIG. 13 is a diagram showing the structure of the laser amplifierto which the pumping module according to embodiment 4 of the presentinvention is applied. In the figure, the same reference numerals asshown in FIGS. 1, 9, and 10 denote the same components or likecomponents.

[0211] Next, a description will be made as to the operation of the laseramplifier.

[0212] When incident light linearly polarized in the direction of the xaxis passes through the polarizer 47, the light travels along theforward path a within the pumping module while it is linearly polarizedin the direction of the x axis. The laser light that has returned fromthe pumping module and travels along the backward path b is linearlypolarized in the direction of the y axis because of the orthogonalaction of the pumping module. The laser light linearly polarized in thedirection of the y axis is reflected in a direction perpendicular to theoptical axis 26 by the polarizer 47, and emerges from the laseramplifier as an emerging laser light amplified by the pumping module.

[0213] Thus, the laser amplifier of FIG. 13 isolates the incident lightand the emerging light from each other by using the polarizer 47according to the fact that the laser light traveling along the forwardpath a has a polarization perpendicular to that of the laser lighttraveling along the backward path b.

[0214] The above-mentioned operation of the laser amplifier issummarized as follows: incident light (linearly polarized in thedirection of the x axis)→passing through the polarizer 47→travelingalong the forward path a (linearly polarized in the direction of the xaxis)→further traveling forwards and then backwards in the pumpingmodule→traveling along the backward path b (linearly polarized in thedirection of the y axis)→reflected by the polarizer 47→emerging light(linearly polarized in the direction of the y axis).

[0215] As previously explained, the pumping module having the reflectionoptical system includes one eleventh square rod 41 and one twelfthsquare rod 42. However, this embodiment 4 is not limited to thisconfiguration. In other words, both the number of eleventh square rods41 and the number of twelfth square rods 42 are not limited to 1. Equalnumbers of eleventh square rods 41 and twelfth square rods 42 can beprovided so that they are cascaded. This variant provides the sameadvantages.

[0216] In the case of this cascade connection, equal numbers of squarerods in which heat is dissipated in the direction of the x axis andsquare rods in which heat is dissipated in the direction of the y axisonly have to be provided and the locations of the equal numbers ofsquare rods in which heat is dissipated in the direction of the x axisand square rods in which heat is dissipated in the direction of the yaxis are not particularly restricted and can be arbitrarily determined.

[0217] For example, when the pumping module is provided with one squarerod in which heat is dissipated in the direction of the x axis and onesquare rod in which heat is dissipated in the direction of the y axis,the arrangement of the two square rods can include the following twopatterns according to the direction of heat sinking when viewed from theincident side of light: a first pattern; the direction of the x axis andthe direction of the y axis (X·Y), and a second pattern; the directionof the y-axis and the direction of the x axis (Y·X). When the pumpingmodule is provided with two square rods in which heat is dissipated inthe direction of the x axis and two square rods in which heat isdissipated in the direction of the y axis, the arrangement of the foursquare rods can be any one of the following six patterns according tothe direction of heat sinking when viewed from the incident side oflight: X·X·Y·Y, X·Y·X·Y, X·Y·Y·X, Y·Y·X·X, Y·X·Y·X, and Y·X·X·Y.

[0218] In a case where the pumping module is provided with three, four,or five, . . . square rods in which heat is dissipated in the directionof the x axis and three, four, or five, . . . square rods in which heatis dissipated in the direction of the y axis, the four or more squarerods can be similarly configured.

[0219] In addition, as explained in Embodiment 1, each of the eleventhand twelfth square rods 41 and 42 can be constructed so that the laserlight propagates along a zig-zag optical path which is bent severaltimes in the direction of heat sinking. In this case, the sameadvantages are provided.

[0220] Furthermore, it is also possible to apply the technique asdisclosed in Embodiment 2 to the pumping module of this embodiment 4. Inother words, the pumping module can be constructed of one square rod inwhich an eleventh square rod 41 and a twelfth square rod 42 areintegrally formed and from one half of which heat is dissipated in adirection perpendicular to that in which heat is dissipated from theremaining half of the single square rod, a first 45-degree polarizationrotator, and a first total reflection mirror. This variant offers thesame advantages.

[0221] At least one integrally formed square rod is adequate for formingthe pumping module when heat is dissipated from one half of theintegrally formed square rod in a direction perpendicular to that inwhich heat is dissipated from the remaining half of the single squarerod. In this case, the pumping module is constructed of one or more suchsquare rods, a first 45-degree polarization rotator, and a totalreflection mirror. Each of the one or more square rods is so constructedthat heat is dissipated from one half of each square rod in a directionperpendicular to that in which heat is dissipated from the remaininghalf of each square rod. The order of the directions of heat sinking isnot limited, and the number of square rod halves in which heat isdissipated in the direction of the x axis only has to be equal to thenumber of square rod halves in which heat is dissipated in the directionof the y axis.

[0222] As mentioned above, in accordance with this embodiment 4, thepumping module is provided with a reflection square rod group includingone or more eleventh square rods 41 having an optical axis 26 and eachhaving a couple of heat sinking surfaces which are normal to a directionof y axis perpendicular to the optical axis 26 and the same number oftwelfth square rods 42 as that of eleventh square rods 41, having theoptical axis 26 in common with the one or more eleventh square rods andeach having a couple of heat sinking surfaces which are normal to adirection of x axis perpendicular to the optical axis 26 and thedirection of the y axis; a first total reflection mirror 44 forreflecting laser light emitted out of the reflection square rod grouptowards the reflection square rod group; and a first 45-degreepolarization rotator 43 having the optical axis in common with the oneor more eleventh square rods and disposed between the reflection squarerod group and the first total reflection mirror 44, for rotating apolarization of the laser light passing therethrough by 45 degrees. As aresult, the present embodiment offers an advantage of being able toprevent any decrease in the extinction ratio regardless of a variationin the orientation of the birefringence axes of each of the plurality ofsquare rods, and to prevent the occurrence of a difference between athermal lens with respect to the direction of the x axis and anotherthermal lens with respect to the direction of the y axis, therebypreventing the occurrence of astigmatism.

[0223] Furthermore, in accordance with this embodiment 4, the reflectionsquare rod group is provided with the one or more eleventh square rods41 and the one or more twelfth square rods 42 in each of which the laserlight is allowed to propagate along a zig-zag optical path between thecouple of heat sinking surfaces thereof. As a result, the presentembodiment offers an advantage of being able to make the temperaturegradients caused in the direction of heat sinking uniform in each of theplurality of square rods, thereby preventing thermal lens effects frombeing produced.

[0224] In addition, in accordance with this. embodiment 4, equal numbersof the one or more eleventh square rods 41 and the one or more twelfthsquare rods 42 are integrally formed in the reflection square rod group.As a result, the present embodiment offers an advantage of being able tofacilitate the alignment of each of the plurality of square rods and toomit processes such as grinding and coating of each of the plurality ofsquare rods, thereby reducing the cost of the pumping module.

[0225] Furthermore, in accordance with this embodiment 4, a laseroscillator is provided with the pumping module and a partial reflectionmirror 45 that pairs up with a first total reflection mirror 44 of thepumping module and is disposed so that a reflection square rod group anda first polarization rotator 43 are sandwiched between the first totalreflection mirror of the pumping module and the partial reflectionmirror and that is perpendicular to an optical axis 26 of the pumpingmodule. As a result, the present embodiment offers another advantage ofbeing able to prevent any decrease in the efficiency of energy and anydecrease in the beam quality and providing a laser oscillator that doesnot have to use a mechanism of compensating for astigmatism.

[0226] In addition, in accordance with this embodiment 4, the laseroscillator includes a second 45-degree polarization rotator 46 havingthe optical axis 26 in common with the pumping module and disposedbetween the partial reflection mirror 45 and the pumping module, forrotating a polarization of laser light passing therethrough by 45degrees. As a result, the present embodiment offers another advantage ofbeing able to prevent any decrease in the efficiency of energy and anydecrease in the beam quality and providing a laser oscillator that doesnot have to use a mechanism of compensating for astigmatism.

[0227] Furthermore, in accordance with this embodiment 4, the laseroscillator includes a polarizer 47 disposed on the optical axis 26between the partial reflection mirror 45 and the pumping module, forallowing laser light linearly polarized in the direction of the x axisto pass therethrough, and for reflecting laser light linearly polarizedin the direction of the y axis perpendicular to the polarization in thedirection of x axis of the former laser light in a directionperpendicular to the optical axis 26, and a second total reflectionmirror 48 for reflecting the laser light reflected by the polarizer 47towards the polarizer 47. As a result, the present embodiment offersanother advantage of being able to prevent any decrease in theefficiency of energy and any decrease in the beam quality and providinga laser oscillator that does not have to use a mechanism of compensatingfor astigmatism.

[0228] In addition, in accordance with this embodiment 4, a laseramplifier includes: a pumping module and a polarizer 47 disposed on theoptical axis 26 of the pumping module, for allowing laser light linearlypolarized in the direction of the x axis to pass therethrough, and forreflecting laser light linearly polarized in the direction of the y axisperpendicular to the polarization in the direction of x axis of theformer laser light in a direction perpendicular to the optical axis 26,and the laser light linearly polarized in the direction of the x axis isinput to the pumping module by way of the polarizer 47. As a result, thepresent embodiment offers another advantage of being able to prevent anydecrease in the efficiency of energy and any decrease in the beamquality and providing a laser amplifier that does not have to use amechanism of compensating for astigmatism.

INDUSTRIAL APPLICABILITY

[0229] As mentioned above, a pumping module in accordance with thepresent invention is suitable for a laser oscillator and a laseramplifier for use in spaceborne laser equipment and laser equipmentintended for machining.

1. A pumping module for making laser light pass through square rods eachhaving a couple of heat sinking surfaces opposite to each other andshaped like a square pillar so as to provide a gain for the laser light,characterized in that said pumping module comprises: a first square rodgroup including a first square rod having an optical axis and having acouple of heat sinking surfaces normal to a first axis perpendicular tosaid optical axis and a second square rod having said optical axis incommon with said first square rod and having a couple of heat sinkingsurfaces normal to a second axis perpendicular to said optical axis andsaid first axis; a second square rod group including a third square rodhaving said optical axis in common with said first square rod and havinga couple of heat sinking surfaces normal to said first axis and a fourthsquare rod having said optical axis in common with said first square rodand having a couple of heat sinking surfaces normal to said second axis;and a 90-degree polarization rotator disposed between said first andsecond square rod groups and having said optical axis in common withsaid first through fourth square rods, for rotating a polarization ofsaid laser light passing therethrough by 90 degrees.
 2. The pumpingmodule according to claim 1, characterized in that said first square rodgroup is provided with said first square rod and said second square rodin each of which the laser light is allowed to propagate along a zig-zagoptical path between the couple of heat sinking surfaces thereof, andsaid second square rod group is provided with said third square rod andsaid fourth square rod in each of which the laser light is allowed topropagate along a zig-zag optical path between the couple of heatsinking surfaces thereof.
 3. A pumping module characterized in that saidmodule comprises a plurality of pumping modules according to claim 1 andsaid plurality of pumping modules are arranged so that their opticalaxes coincide with one another and they are cascaded.
 4. The pumpingmodule according to claim 1, characterized in that said first square rodgroup is provided with said first square rod and said second square rodwhich are integrally formed, and said second square rod group isprovided with said third square rod and said fourth square rod which areintegrally formed.
 5. A pumping module for making laser light passthrough square rods each having a couple of heat sinking surfacesopposite to each other and shaped like a square pillar so as to providea gain for the laser light, characterized in that said module comprises:a first polarization rotating optical system including seventh andeighth square rods having an optical axis and each having a couple ofheat sinking surfaces which are normal to a first axis perpendicular tosaid optical axis, and a first 90-degree polarization rotator havingsaid optical axis in common with said seventh and eighth square rods anddisposed between said seventh and eighth square rods, for rotating apolarization of said laser light passing therethrough by 90 degrees; anda second polarization rotating optical system including ninth and tenthsquare rods having said optical axis in common with said seventh andeighth square rods and each having a couple of heat sinking surfaceswhich are normal to a second axis perpendicular to said optical axis andsaid first axis, and a second 90-degree polarization rotator having saidoptical axis in common with said seventh and eighth square rods anddisposed between said ninth and tenth square rods, for rotating apolarization of said laser light passing therethrough by 90 degrees. 6.The pumping module according to claim 5, characterized in that saidfirst polarization rotating optical system is provided with said seventhsquare rod and said eighth square rod in each of which the laser lightis allowed to propagate along a zig-zag optical path between the coupleof heat sinking surfaces thereof, and said second polarization rotatingoptical system is provided with said ninth square rod and said tenthsquare rod in each of which the laser light is allowed to propagatealong a zig-zag optical path between the couple of heat sinking surfacesthereof.
 7. A pumping module characterized in that said module comprisesa plurality of pumping modules according to claim 5 and said pluralityof pumping modules are arranged so that their optical axes coincide withone another and they are cascaded.
 8. A pumping module for making laserlight pass through square rods each having a couple of heat sinkingsurfaces opposite to each other and shaped like a square pillar so as toprovide a gain for the laser light, characterized in that said modulecomprises: a reflection square rod group including one or more eleventhsquare rods having an optical axis and each having a couple of heatsinking surfaces which are normal to a first axis perpendicular to saidoptical axis and a same number of twelfth square rods as that ofeleventh square rods, having said optical axis in common with said oneor more eleventh square rods and each having a couple of heat sinkingsurfaces which are normal to a second axis perpendicular to said opticalaxis and said first axis; a first total reflection mirror for reflectingsaid laser light emitted out of said reflection square rod group towardssaid reflection square rod group; and a first 45-degree polarizationrotator having said optical axis in common with said one or moreeleventh square rods and disposed between said reflection square rodgroup and said first total reflection mirror, for rotating apolarization of said laser light passing therethrough by 45 degrees. 9.The pumping module according to claim 8, characterized in that saidreflection square rod group has equal numbers of said one or moreeleventh square rods and said one or more twelfth square rods which areintegrally formed.
 10. The pumping module according to claim 8,characterized in that said reflection square rod group is provided withsaid one or more eleventh square rods and said one or more twelfthsquare rods in each of which the laser light is allowed to propagatealong a zig-zag optical path between the couple of heat sinking surfacesthereof.
 11. A laser oscillator characterized in that said oscillatorcomprises: a pumping module according to claim 1; a total reflectionmirror that is perpendicular to an optical axis of said pumping module;and a partial reflection mirror that is disposed so that said pumpingmodule is sandwiched between said partial reflection mirror and saidtotal reflection mirror and that is perpendicular to said optical axis.12. A laser oscillator characterized in that said oscillator comprises:a pumping module according to claim 5; a total reflection mirror that isperpendicular to an optical axis of said pumping module; and a partialreflection mirror that is disposed so that said pumping module issandwiched between said partial reflection mirror and said totalreflection mirror and that is perpendicular to said optical axis.
 13. Alaser oscillator characterized in that said oscillator comprises: apumping module according to claim 8; and a partial reflection mirrorthat pairs up with a first total reflection mirror of said pumpingmodule and is disposed so that a reflection square rod group and a firstpolarization rotator are sandwiched between said first total reflectionmirror of said pumping module and said partial reflection mirror andthat is perpendicular to an optical axis of said pumping module.
 14. Thelaser oscillator according to claim 13, characterized in that said laseroscillator comprises a second 45-degree polarization rotator having theoptical axis in common with said pumping module and disposed betweensaid partial reflection mirror and said pumping module, for rotating apolarization of laser light passing therethrough by 45 degrees.
 15. Thelaser oscillator according to claim 14, characterized in that said laseroscillator comprises a polarizer disposed on the optical axis betweensaid partial reflection mirror and said pumping module, for allowinglaser light of a predetermined polarization to pass therethrough, andfor reflecting laser light of a polarization perpendicular to the formerlaser light of the predetermined polarization in a directionperpendicular to said optical axis, and a second total reflection mirrorfor reflecting the laser light reflected by said polarizer towards saidpolarizer.
 16. A laser amplifier characterized in that said amplifieramplifies an input laser light by using a pumping module according toclaim
 1. 17. A laser amplifier characterized in that said amplifieramplifies an input laser light by using a pumping module according toclaim
 5. 18. A laser amplifier characterized in that said amplifiercomprises: a pumping module according to claim 8; and a polarizerdisposed on an optical axis of said pumping module, for allowing laserlight of a predetermined polarization to pass therethrough, and forreflecting laser light of a polarization perpendicular to the formerlaser light of the predetermined polarization in a directionperpendicular to said optical axis, the laser light of the predeterminedpolarization being input to said pumping module by way of saidpolarizer.